What about having an average limit stating that the British rider stays at reserve until his average is over a certain value and providing his average is in the top 5.
Another rule like this wouldn't make it whole lot more complicated than it already is.
I was just trying to think of a rule that would benefit British riders as there are PL teams that don't have British riders in their lineups and there are a few NL riders that could line up at reserve for them.